Question

% cat temp
$$$ hello1
$$  hello2
    hello3
##  hello4
    hello5 $$$
% cat temp | grep "$$$"
Illegal variable name.
% cat temp | grep "\$\$\$"
Variable name must contain alphanumeric characters.
%

I want to grep for $$$ and I expect the result to be

% cat temp | grep <what should go here?>
$$$ hello1
    hello5 $$$
%

To differentiate, I have marked the prompt as %.

• What is the problem here?
• What should the grep string be?

Answer 1

The problem is that the shell expands variable names inside double-quoted strings. So for "$$$" it tries to read a variable name starting with the first $.

In single quotes, on the other hand, variables are not expanded. Therefore, '$$$' would work – if it were not for the fact that $ is a special character in regular expressions denoting the line ending. So it needs to be escaped: '\$\$\$'.